JC virus (JCV) is the etiologic agent of progressive multifocal leucoencephalopathy (PML), a lyric infection of oligodendrocytes occurring in patients with underlying immunosuppression. JCV infects 90% of normal adults, but does not cause any disease in healthy individuals. Viral reactivation occurs in 4% of patients with AIDS, and occasional other individuals who are profoundly immunosuppressed. This reactivation leads to a lyric infection of oligodendrocytes and an associated demyelination of the central nervous system. The nature of the immune response that contains the virus in immunocompetent people, and fails in immunosuppressed individuals, is unknown. Anti-JCV antibodies do not protect against PML. Therefore, the JCV-specific cellular immune response is likely to have a crucial role in containing replication of this virus. Understanding the nature of the cellular immune response to JCV is of central importance in characterizing the immunopathogenesis of PML. Cytotoxic T lymphocytes (CTL) are the major effector cells involved in containing many viral infections. CTL recognize virus-infected cells through the interaction of their T-cell receptor and a viral peptide epitope presented by the MHC class I molecule of the infected cell. Virus-specific CTL can be detected using functional cytotoxicity assays, by flow cytometric analyses through staining of lymphocytes with tetrameric MHC class I/viral peptide complexes, and in antigen-stimulated cytokine production assays. In the studies described in this application we will characterize the JCV-specific CTL response in humans and assess the role of this immune response in the immunopathogenesis of PML. Specifically, we will 1) characterize JCV-specific cytotoxic T lymphocytes using functional lysis assays 2) map JCV CTL epitopes in patients with PML 3) develop JCV-specific T lymphocyte ELISPOT assays 4) construct MHC class I/JCV peptide tetramer complexes and determine JCV CTL epitope immunodominance 5) analyze the cellular immune response to JCV in patients with PML and control subjects, using MHC class I/JCV peptide tetramer complex staining and ELISPOT assays 6) define the cellular immune response to JCV in the cerebrospinal fluid 7) correlate the detection of JCV-specific CTL with the immune status and clinical course of patients with PML